1. Field of the Invention
The present invention relates to an image-reading apparatus that detects the size of a document placed in a prescribed position and reads an image from the document, in the document size detected.
2. Description of the Related Art
Image-reading apparatuses for use in copying apparatus or scanners have been developed. Most of them have the function of automatically detecting the size of a document placed at a prescribed position.
An image-reading apparatus of this type has a plurality of reflection-type photosensors each including a light-applying element and light-receiving element. These photosensors are arranged at specified positions below the document table on which a document is mounted. The light-applying elements are connected to a pulse-wave oscillator circuit that generates pulses at prescribed intervals. Hereinafter, the combination of the reflection-type photosensors and the pulse-wave oscillator circuit, both used to detect the size of a document, will be referred to as “document-size detecting sensor.” The document-size detecting sensor is connected to a CPU that controls the reading of images. The CPU determines whether the reflection-type photosensors have detected a document or not. From the outputs of the photosensors the CPU detects the size of a document, if any.
Generally, the image-reading apparatus receives electrical power from a 3.3V or 5V power supply designed to drive control logic ICs or a 24V power supply designed to drive electrical motors and exposure lamps. The apparatus may need to operate on not only this power supply, but also another power supply. In this case, a regulator or the like is connected to the power supply to provide, if necessary, the other power supply, in order to avoid an increase in the power-supply cost. Hence, only the power-supply voltage specified above is applied from a switching power supply. This is why the control logic ICs incorporated in the image-reading apparatus operate on the same power supply. Since the document-size detecting sensor receives power from the switching power supply, it is turned on or off in accordance with the supply of power to or the interruption of the supply of power to the image-reading apparatus. More specifically, power is supplied to the document-size detecting sensor as long as the power supply of the image-reading apparatus remains on.
In the document-size detecting sensor, the pulse-wave oscillator circuit controls the light-applying elements. Ripples may therefore occur in the power supply of the light-applying elements. The power supply works also as power supply to the control logic ICs provided in the image-reading apparatus. If ripples occur in the power supply, they greatly influence the other control units connected to this power supply, such as the amplifier (AMP) unit, A/D unit, shading-correcting unit and image-processing unit. Of these units, the amplifier unit is most influenced. This is because this unit amplifies the analog signal output from the CCD incorporated in the image-reading apparatus. Ripples, if any in the power supplied to the amplifier unit, are superimposed on the image signal, which will represent an image that contains moiré.